The present invention relates to a useful technique applied to an electro luminescence device and a method for producing the same, such as a LED (light emitting diode) or a LD (laser diode) produced by employing a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in the periodic table.
With compound semiconductors that comprise a Group 12 (2B) element and a Group 16 (6B) element in the periodic table (hereinafter, that are referred to Group II-VI compound semiconductors), generally, free control of conduction types of p-type and n-type is difficult except CdTe (cadmium telluride). Thus, extremely a few electro luminescence devices provided with these materials and methods for producing the same are made practicable, and the ranges thereof remain limited.
For example, with a method for fabricating a light emitting diode as an electro luminescence device by using a ZnSe system material, a large number of mixed crystal thin films of ZnSe system are formed on a GaAs substrate by a molecular beam epitaxial growth method, thereafter electrodes are formed, and then a pn junction type light emitting diode is fabricated.
When fabricating the light emitting diode, with the ZnSe system material, since the control for a p-type semiconductor is difficult in a thermal equilibrium state, the epitaxial growth method which is not in the thermal equilibrium state was applied to formation of the mixed crystal thin films by using a particular apparatus which is referred to as a radical gas source.
As an electro luminescence device provided with such a ZnSe system material, for example, 480 nm blue LEDs are manufactured by way of trial. Furthermore, fabrication of blue LDs in quantum well structure of CdZnSexe2x80x94ZnSe is reported, and it draws attention as a blue light emitting device.
However, as above-described, with the electro luminescence device provided with the Group II-VI compound semiconductor, the material system is extremely limited because the physical property that the control of conduction types in the Group II-VI compound semiconductor is difficult. Thus, the electro luminescence device having the Group II-VI compound semiconductor has not bean put to practical use except for the ZnSe system materials.
When the electro luminescence device provided with the ZnSe system material was fabricated, the epitaxial growth method was required to be applied to the fabrication, to make control of the conduction types possible. Thus, there were problems that the productivity was low, and that the production cost increased because an expensive apparatus such as the radical gas source or the like was required.
Then, the inventors or the like proposed a method for forming an electro luminescence device by using the Group II-VI compound semiconductor single crystal substrate, and forming a pn junction by thermally diffusing a diffusion source including an element converting the substrate of a first conduction type into one of a second conduction type from a front surface of the substrate.
However, there was a problem that the characteristics of the electro luminescence device fabricated by the method depended heavily on quality of the used substrate, and thus the electro luminescence device having superior light emission efficiency was not stably fabricated.
The present invention was developed to solve the above-described problems. A main object of the present invention is to provide a method that is capable of stably producing an electro luminescence device having superior light emission efficiency by using a Group II-VI compound semiconductor crystal substrate.
At first, the inventors investigated depositing diffusion sources over ZnTe substrates of compound semiconductors (Group II-VI compound semiconductors) comprising Group 12 (2B) elements and Group 16 (6B) elements in the periodic table and being produced by some producing methods, and then formed pn junctions by thermally diffusing the diffusion sources. Thereafter, the inventors investigated the correlation between the light emission characteristics and the qualities of the substrate (particularly, crystal dislocation).
As a result, green light emission was able to be recognized from light emitting diodes produced by using substrates on which density of pits (hereinafter, it referred to as etch pits), which were obtained by etching with high temperature sodium hydroxide aqueous solution, was not more than 20,000/cm2, preferably not more than 10,000/cm2, more preferably not more than 5,000/cm2, furthermore, not more than 2,000/cm2 
On the other hand, with light emitting diodes produced by using substrates on which the density of the etch pits exceeded 20,000/cm2, no light emission was able to be recognized.
It was verified that the etch pits formed using sodium hydroxide occurred due to the dislocation in the crystal by another experiment. Therefore, with the ZnTe substrate, the dislocation density and etch pit density can be treated equally.
From the result of the above-described researches, it was ascertained that the light emitting phenomenon of the light emitting diode depends largely on the dislocation density or the etch pit density of the front surface of the substrate.
It has been known that a large number of inclusions exist inside of crystals of the Group II-VI compound semiconductor depending upon growth methods or growth conditions. For example, the Group II-VI compound semiconductor, which is applied to a substrate for a visible light emitting diode, has wide forbidden band width and is transparent. Thus, the inclusions inside the substrate can be observed by an optical microscope.
Thus p-type ZnTe substrates that were different from each other in densities of inclusions were prepared. Then, as a diffusion source, for example, Al or In was deposited over the front surfaces of the substrates, and pn junctions were formed by the thermal diffusion. The characteristics of the light emitting diodes formed by such a method were compared with one another. When the density of the inclusions having grain diameters of 0.3 to 10 xcexcm on the pn junction interfaces, which were observed in a focal field of the optical microscope of xc3x97100 to xc3x97200 magnification, was not more than 100,000/cm2, preferably not more than 50,000/cm2, it was possible to obtain the light emitting diodes having a little leakage current due to recombination and superior light emission efficiency.
On the other hand, when the density of the inclusions exceeded 100,000/cm2, the light emission efficiency decreased. In particular, in the substrate having inclusions which were larger than 5 xcexcm, even if the density of the inclusions was in single figure smaller 10,000 to 50,000/cm2, it was found that the leakage current increased and the light emission efficiency lowered.
Consequently, it is considered that the leakage current occurs because the inclusions in the pn junction interface form current passages.
Therefore, it is supposed that suppression of the inclusions in the pn junction interfaces plays a role for decreasing the leakage current and thus improving light emission efficiency.
From a result of observation by a scanning electron microscope, the number of the inclusions existed in the interface is generally smaller than the number of the inclusions observed by the optical microscope.
This depends on the sizes of the inclusions. When the sizes of the inclusions are about 1 xcexcm, the density of the inclusions in the interface and the density of the inclusions observed by the optical microscope are at the same level, while when the sizes of the inclusions are small, the density of the inclusions in the interface is about in single figure small compared with the density of the inclusions observed by the optical microscope.
Then, as a result of the researches, when the number of the inclusions existed in the junction interface was not more than 50,000/cm2, it was possible to obtain the electro luminescence device in which the leakage current due to the recombination is small and having superior efficiency.
The first invention according to the present inventions was developed based on the above-described findings, and is an electro luminescence device comprising a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in a periodic table, wherein the electro luminescence device is produced by providing a substrate having a low defect density, and forming a pn junction in the vicinity of a front surface of the substrate by thermally diffusing an element converting the substrate of a first conduction type into the one of a second conduction type from the front surface of the substrate.
According to the invention, the leakage current due to the recombination can be reduced, and the electro luminescence device (for example, light emitting diode which emits green light) having high light emission efficiency can be stably obtained.
When the conduction type of the substrate (first conduction type) is p-type, the element thermally diffused is impurity (donor) converting the substrate into the one of n-type, while when the conduction type of the substrate is n-type, the element thermally diffused is impurity (acceptor) converting the substrate into the one of p-type.
As the substrate, the one on which density of pits obtained by etching with sodium hydroxide aqueous solution at 90 to 130xc2x0 C. is not more than 20,000/cm2, more preferably not more than 10,000/cm2, particularly preferably not more than 5,000/cm2, furthermore, not more than 2,000/cm2, may be used.
With a ZnTe substrate, the dislocation density and the density of etch pits occurred due to the sodium hydroxide can be treated equally. Thus, as the substrate, the one on which the dislocation density is not more than 20,000/cm2, more preferably not more than 10,000/cm2, particularly preferably not more than 5,000/cm2, furthermore, not more than 2,000/cm2, may be used. Alternatively, because the density of the etch pits occurred when etching the substrate correlates with the dislocation density in the substrate, density of etch pits occurred by other etchants may be used as a condition.
As the substrate, the one in which the density of the inclusions having grain diameters of 0.3 to 10 xcexcm in the pn junction interfaces and being able to be observed in a focal field of the optical microscope of xc3x97100 to xc3x97200 magnification is not more than 100,000/cm2, may be used.
Furthermore, the substrate may be made of any one of ZnTe, ZnSe and ZnO.
According to the means, the electro luminescence device can be obtained wherein the wavelengths of the light emitted from the opposite light emitting regions sandwiching an interface of the pn junction are different from each other.
More concretely, with the electro luminescence device produced by providing a p-type ZnTe as the substrate, and using Al, Ga, In, or alloy including them as the diffusion source including an element converting the substrate of the first conduction type into the one of the second conduction type, the light emitted from the light emitting region in a side of the diffusion source is from green light to red light having a wavelength of from 550 to 700 nm, while the light emitted from the light emitting region in a side of the substrate is from yellow light to red light having a wavelength of from 580 to 700 nm. The light emitting regions sandwich the interface of the pn junction.
Details are shown in Table 1.
Furthermore, the inventors made repeated investigations into a method for controlling the conduction type of the Group II-VI compound semiconductor. Then, it was reasoned that when the impurities are doped into the crystal by diffusion, if the formation of vacancies can be controlled at the diffusion step, the effect of selfcompensation may be suppressed and efficient control of the conduction type may be possible.
As a result of repeated investigations based on the reasoning, the fruits were obtained as follows. That is, when diffusion source is disposed on a front surface of a Group II-VI compound semiconductor substrate of a first conduction type, the diffusion source including an element converting the substrate into one of a second conduction type, and then is thermally diffused, it can prevent that the highly volatile constitute element in the substrate is come off from the front surface of the substrate during the diffusion step, so that it is possible to prevent vacancies from forming.
As regards impurities remaining on the front surface of the substrate, it was found that when a compound of the element included in the diffusion source and the impurities is more stable at the diffusion temperature than a compound of the element constituting the substrate and the impurities, the impurities can be removed from the front surface of the substrate, so that it gives the effect of improving the purity of the front surface of the substrate.
Then, based on the fruits of the investigations, the experiment was carried out using Al or In (which may be in the form of impurities wherein a p-type ZnTe substrate is converted into one of n-type. The Al or In was deposited under vacuum on a front surface of the substrate to form Al or In thin film, and then heat treatment was carried out under N2 atmosphere.
As a result, it was found that the deposited Al or In can prevent the highly volatile Zn from vaporizing from the front surface of the substrate, giving the effect of suppressing formation of vacancies in the substrate.
Further, since Al or In forms a stable compound with impurities, such as oxygen or the like, in the ZnTe substrate, it is expected that Al or In removes the impurities from a front surface layer of the substrate to improve the purity of the front surface of the substrate.
Then, ohmic electrodes were formed on both surfaces of the substrate into which Al or In was thermally diffused, thereby a light emitting diode as an electro luminescence device was manufactured by way of trial. This light emitting diode showed rectifying characteristics, so that it was possible to recognize the light emission. The formation of pn junction by the method for thermally diffusing Al or In was also verified with EBIC method (Electron Beam Induced Current Method).
Accordingly, it was proved that this method is effective for forming a pn junction of the Group II-VI compound semiconductor.
As a result of comparing between cases that Al and In are used as the diffusion sources, it was found that when Al is used, the light emission color is nearer to green compared with the case that In is used. Furthermore, it was found that when In is used, red light emission is mixed. This red light emission is considered to be the light emission caused mainly by oxygen impurities. That is, it is known that the oxygen taken in the crystal enters in a Te lattice site location and emits light in red. It is considered that the oxygen exits in the crystal in the form bonded with Zn.
Al and In are strongly bonded with oxygen, and Gibbs"" free energy thereof are xe2x88x921,690 kJ/mol and xe2x88x92635 kJ/mol, respectively, at an anneal temperature of around 600xc2x0 C. The Gibbs"" free energy of them is small and stable compared with the Gibbs"" free energy of ZnO (xe2x88x92260 kJ/mol). Furthermore, because oxide of Al is more stable than oxide of In, Al has a large effect on gettering oxygen from the ZnTe substrate. Thus, the red light emission resulted from oxygen was not generated.
C, Si, Bi or the like, of which oxide has small free energy, can be expected to have the same effect.
As impurities having luminescence peak on the side of a long wavelength in the Group II-VI compound semiconductor, Au, Ag, Cu, Li or the like are given other than oxygen.
Since compounds of Au, Ag and Cu and halogens, such as Cl or the like, are more stable than compounds of Au, Ag and Cu and Zn, these impurities can be removed from the substrate in the diffusion step by using diffusion sources including halogens.
With regard to the heat treating temperature of the diffusion step, as a result of various experiments at the temperature range of from 300xc2x0 C. to 700xc2x0 C., it was found that more uniform diffusion was possible in a low temperature region, and the heat treatment in the range of 300 to 430xc2x0 C. was preferable.
As a result of various experiments in which the heat treatment time was changed in the range of from few minutes to tens of hours, the heat treatment time might be enough if it was not less than the defined time for Al and In, respectively. However, when the diffusion source did not remain on the front surface of the substrate at the end of the diffusion step, it was found that excellent current/voltage characteristics was not obtained, and no light emission was generated in many cases.
The reason of this is considered that when the diffusion source did not remain on the front surface of the substrate with an enough thickness at the end of the diffusion, the diffusion source did not enable sufficient suppression of the evaporation of Zn and thus of the formations of defects, such as vacancies or the like in the substrate. Also, it is considered that gettering impurities, such as oxygen or the like, in the substrate by the diffusion source was not sufficient. Therefore, it was found that remaining of the diffusion source of enough thickness on the front surface of the substrate at the end of the diffusion was important.
The second invention according to the present inventions was developed based on the above-described findings, and is a method for producing an electro luminescence device, comprising the steps of: providing a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in a periodic table; forming a pn junction in the vicinity of a front surface of the substrate by thermally diffusing diffusion source including an element converting the substrate of a first conduction type into the one of a second conduction type; and forming electrodes on both surfaces of the substrate; wherein the diffusion source is disposed on the front surface of the substrate, preventing forming of a defect compensating an impurity level which is formed in the substrate by the element included in the diffusion source during a diffusion step, and furthermore the diffusion source includes an element gettering impurity on the front surface of the substrate.
According to the method, the controllability for conduction type of the Group II-VI compound semiconductor can be improved efficiently by suppressing the effect of selfcompensation and the purity of the front surface of the substrate can also be improved. Thus, the electro luminescence device having superior light emission efficiency can be obtained.
When the conduction type of the substrate (first conduction type) is p-type, the element included in the diffusion source is impurity (donor) converting the substrate into one of n-type, while when the conduction type of the substrate is n-type, the element included in the diffusion source is impurity (acceptor) converting the substrate into one of p-type.
The defect compensating a level showing the conduction type (the second conduction type) which is different from the conduction type (the first conduction type) of the substrate may include a vacancy or a defect including the vacancy.
Furthermore, the diffusion source may comprise such a material that the Gibbs"" free energy of a compound which is formed by combining the diffusion source and impurity in the substrate may be smaller than the Gibbs"" free energy of a compound which is formed by combining the constitute element in the substrate and the impurity at the diffusion temperature.
Alternatively, the diffusion source may comprise such an element that the Gibbs"" free energy of a compound which is formed by combining an element included in the diffusion source and impurity in the substrate may be smaller than the Gibbs"" free energy of a compound which is formed by combining the constitute element in the substrate and the impurity at the diffusion temperature.
The impurity in the substrate is at least one of O, Li, Ag, Cu and Au.
As the diffusion source, Al, Ga, In, or alloy thereof, or Cl, Br, I, or alloy thereof may be used.
The element included in the diffusion source and gettering the impurity in the substrate may comprise an element having a slow diffusion rate into the substrate compared with the element converting the substrate of the first conduction type into one of the second conduction type.
The element included in the diffusion source and gettering the impurity in the substrate may be at least one of B, Si and C.
The diffusion source may be deposited over the front surface of the substrate by any one of a sputtering method, a resistance heating method, and an electron beam method, under vacuum.
The heat treating temperature at the diffusion may be 300xc2x0 C. to 700xc2x0 C., preferably.
The diffusion source may be formed in the film thickness of 1,000 to 10,000 xc3x85, preferably, 1,500 to 5,000 xc3x85.
It may be preferable that the diffusion source may remain on the front surface of the substrate with a predetermined thickness after the diffusion.
The diffusion source may be remained in the thickness of not less than 100 xc3x85, preferably, not less than 300 xc3x85 after the diffusion.
Preferably, when the diffusion source is Al or In, the diffusion time may be longer than the time specified by a relational expression Y=2xc3x97105exp(xe2x88x920.018 T), showing the relation between the diffusion time Y and the heat treating temperature T.
Preferably, the substrate may be ZnTe.
Furthermore, the inventors investigated about light emission characteristics of the electro luminescence device, and found that the light emission characteristics depends strongly on plane orientation of the substrate on which the diffusion source is disposed.
Then, the electro luminescence devices were fabricated by disposing the diffusion sources on various plane orientations of the substrates, and experiments were carried out repeatedly.
Specifically, ZnTe crystal, which is one of the Group II-VI compound semiconductor single crystal, was sliced at various plane orientations, and substrates were obtained. Then, the diffusion source of Al was deposited over the front surface of the substrate and the pn junction was formed by the thermal diffusion. Thereafter, electrodes were provided on both surfaces of the substrates, so that the electro luminescence devices were fabricated. The light emission characteristics of these specimens were investigated.
As a result, with the specimens each of which a substrate plane is the one other than (111)Te plane, the light emission was recognized from almost whole planes of the substrates, while from the specimen of which a substrate plane is the (111)Te plane, only weak light emission was recognized.
For finding out the reason, the (111)Te plane and other planes were compared with each other. As a result, plane roughness was occurred in the (111)Te plane after etching of the substrate. On the other hand, no plane roughness was occurred in (111)Zn plane, (001) plane, and (011) plane other than the (111)Te plane after etching.
When Al as the diffusion source was deposited over the front surface in which the plane roughness was occurred, adhesiveness between Al and the front surface of the substrate was poor. Accordingly, it is considered that the thermal diffusion into the substrate was only locally occurred. Actually, a phenomenon of easily peeling of the diffusion source deposited over the (111)Te plane was observed.
The condition of the diffusion was investigated inplane. As a result, with the specimen of which substrate plane was the (111)Te plane, diffusion depth was not uniform but varied largely. Further, many non-diffused portions were observed. On the other hand, with the specimens of which substrate planes were the one other than the (111)Te plane, it was found that the diffusion sources were almost uniformly diffused.
Accordingly, a finding was obtained that plane roughness in the front surface of the substrate causes the ununiformity of diffusion to deteriorate the light emission characteristics. Therefore, it came to a conclusion that for obtaining excellent light emission characteristics, carrying out the etching at least in a condition that no plane roughness is occurred is important.
Consequently, although various etchants for causing no plane roughness were tried, no suitable etchant for the (111)Te plane was found.
On the other hand, it was confirmed that etching the planes other than the (111)Te plane by etchants, such as hydrobromic acid or Br-methanol system, enables obtaining relatively flat surface-condition.
With the substrates of which substrate planes were inclined within 10 degrees from the (111)Zn plane, (001) plane, or (011) plane, flat surfaces without plane roughness were able to be obtained by etching with the etchants, such as hydrobromic acid or Br-methanol system.
The third invention according to the present inventions was developed based on the above-described findings, and is a method for producing an electro luminescence device, comprising the steps of: providing a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in a periodic table; disposing a diffusion source on a front surface of the substrate, the diffusion source including an element converting the substrate of a first conduction type into the one of a second conduction type; forming a pn junction in the vicinity of the front surface of the substrate by thermally diffusing the diffusion source; and forming electrodes on both surfaces of the substrate; wherein the diffusion source is disposed on a substrate plane having plane orientation from which a flat plane is obtained after etching.
According to the invention, avoiding the influence of the plane orientation on the light emission characteristics by limiting the plane orientation of the substrate, the electro luminescence device having superior light emission efficiency can be stably produced.
The substrate may be one of ZnTe, ZnSe, and ZnO, preferably.
When the substrate of which substrate plane is (111)Zn plane, (001) plane, or (011) plane is employed, flat plane may be obtained after etching.
Alternatively, when the substrate of which substrate plane inclines within 10 degrees from the (111)Zn plane, (001) plane, or (011) plane is employed, flat plane may also be obtained after etching.
In addition, before the diffusion source is disposed, the front surface of the substrate may be chemically etched. At this time, the etching by etchant of bromic acid system or bromine system may be preferable.
The inventors further investigated repeatedly for the producing method of electro luminescence device. As a result, it was found that when the diffusion process was performed for relatively long times (it is required for the diffusion source to remain after diffusion process) at a low temperature (300 to 550xc2x0 C.), the elements constituting the diffusion source were uniformly diffused, so that the light emission characteristics became stable.
Then, based on inference that if the condition of diffusion process is the same, the light emission characteristics obtained through the diffusion source after the diffusion should be due to the diffusion source, experiments were repeatedly carried out for determining the most suitable deposition conditions of the diffusion source for more stable light emission characteristics.
The condition of the diffusion process is 16 hours at 420xc2x0 C. With the experiments, for the substrate, p-type ZnTe substrate, which is one of the Group II-VI compound semiconductors, was employed, and Al was employed for the diffusion source.
At first, a thin film of the Al diffusion source was formed on the ZnTe substrate by vacuum deposition with a thickness of 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 200 nm or 500 nm. Thereafter, diffusion process was performed for 16 hours at 420xc2x0 C. to thereby form the pn junction. At this time, after the diffusion process under the diffusion conditions, respective diffusion sources with respective thickness remained on the substrates. Thereafter, electrode was formed on the side of the rear surface of the ZnTe substrate, so that the light emitting diode was produced. Then, the correlation between the thickness of the deposited diffusion source and the light emission characteristics of light observed through the diffusion source was investigated.
As a result, it was found that when the film thickness of the deposited diffusion source was 5 to 50 nm, the light observed through the Al diffusion source was green light having high light emission intensity and being stable, while when the film thickness of the deposited diffusion source exceeded 50 nm, the yellow light became stronger than the green light in the relative intensity, and the light emission intensity lowered as a whole.
From this findings, the inventors considered that the luminescence center of yellow is due to defects caused by excess Al. That is, the inventors inferred that as the film thickness of the Al diffusion source increases, the concentration of Al diffusing into the ZnTe substrate increases, thereby the defects caused by Al increases in the ZnTe substrate, so that the intensity of the yellow luminescence increases.
Then, investigations were repeatedly carried out based on the above-described inference. As a result, producing the electro luminescence device having superior light emission characteristics was achieved by limiting properly the film thickness of the diffusion source.
The inventors further investigated about diffusion length of the diffusion source into the substrate and the light emission characteristics of the obtained electro luminescence device. Then, it was found that when the Al diffusion source remains on the ZnTe substrate, the diffusion length depends strongly on the condition of the diffusion process, while it is little influenced by the film thickness of the Al diffusion source. Accordingly, it is considered that if the conditions of the diffusion process are the same, the diffusion lengths of Al become the same, thereby the light emission intensities become the same when the light emitted from the pn junctions reaches the interface between the substrate and the diffusion source.
However, the light emission intensities obtained through the Al diffusion source differed with the film thicknesses of the Al diffusion source. As a result of further repeated investigations, it was found that since transmittance of the Al diffusion source varies with the film thickness of the Al diffusion source, the intensity of the light obtained trough the Al diffusion source varies. Then, when thinning the film thickness of the diffusion source for the light easily to transmit, the green luminescence with high light emission-intensity and stable was obtained.
As described above, when the film thickness of the diffusion source to be deposited was thinned, the light transmittance of the diffusion source remained after diffusion process became extremely high. Thus, without forming new transparent electrode by removing the diffusion source after diffusion process, it was possible to improve the efficiency of taking light out by employing the diffusion source for an electrode.
The fourth invention according to the present inventions was developed based on the above-described findings, and is a method for producing an electro luminescence device, comprising the steps of: providing a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in a periodic table; disposing a diffusion source on a front surface of the substrate, the diffusion source including an element converting the substrate of a first conduction type into the one of a second conduction type; forming a pn junction in the vicinity of the front surface of the substrate by thermally diffusing the diffusion source; and forming electrodes on both surfaces of the substrate; wherein the diffusion source is disposed on the front surface of the substrate in a film thickness of from 5 nm to 50 nm.
Accordingly, since an amount (concentration) of the diffusion source diffused into the substrate can be controlled, it is possible to prevent the light emission characteristics from changing caused by the defects formed in the substrate, the defects being due to the diffusion source. Therefore, an electro luminescence device having superior light emission characteristics can be produced.
Particularly, with the diffusion source, the film thickness of 5 to 20 nm may be more efficient. With this thickness, the diffusion source remained on the front surface of the substrate after the diffusion process may become thin for enough light transmittance, so that the intensity of the light transmitting the diffusion source may become high. Thus, without forming a transparent electrode, such as ITO or the like, the electro luminescence device having superior light take out efficiency may be produced with easy processes and in relatively low cost.
Preferably, the processing temperature for the diffusion may be 300 to 550xc2x0 C. Further, it is preferable that the processing time for the diffusion may be determined so that the diffusion source will remain in a predetermined thickness after the diffusion process, for example, in a thickness of 3 to 15 nm.
Preferably, the substrate may be ZnTe, ZnSe or ZnO. Further, the diffusion source may be Al, Ga, In, or alloy thereof.
Next, the inventors or the like researched relation between the light emission intensity of the electro luminescence device and PL (photoluminescence) intensity of the substrate before the diffusion source is diffused, and found out that there is a strong correlation between both. Accordingly, it was found out that when the electro luminescence device is produced by employing a substrate having high PL intensity, the electro luminescence device having superior light emission characteristics can be obtained.
It was further found out that the PL intensity of the substrate before the diffusion depends strongly on carrier density in the substrate.
Then, for determining the optimum carrier density for high PL intensity of the substrate before the diffusion, investigations about relation between the PL intensity of the substrate before the diffusion and the carrier density were repeatedly carried out.
Concretely, dopant of predetermined amount was doped into the Group II-VI compound semiconductor single crystal substrates, so that substrates having carrier densities of 7xc3x971016 to 7xc3x971018 cmxe2x88x923 were fabricated. Then, the PL intensity of each substrate was measured, and the relation of the PL intensity to the carrier density of the substrate was investigated. The result is shown in FIG. 4. It is understood from FIG. 4 that the PL intensity does not increase in proportion to the carrier density, but decreases when the carrier density is not less than a value. Furthermore, from the above-described finding, it is inferred that when the substrate having the carrier density within a range of 1xc3x971017 to 5xc3x971018 cmxe2x88x923, the electro luminescence device having high light emission intensity can be obtained.
Then, the inventors carried out the following experiments for confirming the above-described inference.
At first, a substrate having carrier density of 7xc3x971016 to 7xc3x971018 cmxe2x88x923 was employed, and the diffusion source was deposited over the front surface of the substrate and thermally diffused, so that the pn junction was formed. Thereafter, an ohmic electrode was formed on the rear surface of the substrate, so that the electro luminescence device was fabricated. The light emission characteristics thereof was investigated.
As a result, it was verified that the electro luminescence device fabricated by employing the substrate having carrier density of 1xc3x971017 to 5xc3x971018cmxe2x88x923 emitted green light with high intensity. Further, the electro luminescence device fabricated by employing the substrate having carrier density of 3xc3x971017 to 2xc3x971018 cmxe2x88x923 emitted stable green light with high intensity. Furthermore, the electro luminescence device fabricated by employing the substrate having carrier density of 5xc3x971017 to 9xc3x971017 cmxe2x88x923 emitted stable green light with higher intensity.
On the other hand, with the electro luminescence device fabricated by employing the substrate having carrier density of less than 1xc3x971017 cmxe2x88x923 or more than 5xc3x971018 cmxe2x88x923, it was verified that the light emission intensity was lower than that of the electro luminescence device fabricated by employing the substrate having carrier density of 1xc3x971017 to 5xc3x971018 cmxe2x88x923.
The fifth invention according to the present inventions was developed based on the above-described findings, and is an electro luminescence device comprising a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in a periodic table, wherein the electro luminescence device is produced by disposing a diffusion source including an element converting the substrate of a first conduction type into the one of a second conduction type on a front surface of the substrate; forming a pn junction in the vicinity of the front surface of the substrate by thermally diffusing the diffusion source; and forming electrodes on both surfaces of the substrate, and wherein the substrate has carrier density of from 1xc3x971017 cmxe2x88x923 to 5xc3x971018 cmxe2x88x923.
Preferably, the substrate may be ZnTe, ZnSe or ZnO. Further, the diffusion source may be Al, Ga, In, or alloy thereof. The substrate may have the desired carrier density by doping determined amount of the Group 15 (5B) element, for example, phosphorus in the periodic table.
Furthermore, the inventors investigated the light emission characteristics of the electro luminescence device. As a result, it was found that most emitted light was absorbed around the front surface of the substrate, so that the light emitted to outside became considerably weak.
One of causes of this is inferred that the ZnTe, ZnSe, ZnO or the like constituting the substrate is a direct transition type. That is, for example, with the ZnTe, an absorption coefficient a for light at 550 nm in band edge luminescence is up to 1xc3x97104/cm. As light damps to 1/e (e=2.73) per a thickness of 1 xcexcm, the emitted light damps exponentially with increasing the thickness of the substrate through which the light passes. Therefore, with the electro luminescence device in which the band edge luminescence is employed, the light should be taken out of the front surface of the substrate before the light damping.
Further investigations were carried out about the above-described point. As a result, a conclusion can be obtained that for increasing the light emission intensity of the electro luminescence device, the diffusion depth is required to fall in a range, in which the light with desired intensity can be taken out.
Then, employing the p-type ZnTe as a substrate, experiments were carried out for determining the optimum diffusion depth when Al was used as the diffusion source.
At first, the Al as the diffusion source was deposited over the p-type ZnTe substrate, and thermally diffused, so that a pn junction was formed. Then, electrodes were formed on front and rear of the substrate, so that the light emitting diode was fabricated. The light emitting diode was evaluated by an EBIC (Electron beam induced current) method. As a result, as shown in a graph in FIG. 5, it was found that the carrier density of the dopant was approximately the same as or slightly lower than the carrier density of the substrate. In addition, it was found that a diffusion length of minority carrier was short, about 0.2 to 0.3 xcexcm.
According to the above-described result, it is supposed that a thickness of a depletion layer formed from the junction portion to an n-type layer may be 0.1 to 0.7 xcexcm within the optimum carrier density of the substrate. For taking out the light emission from the front surface of the substrate, the light emission being caused by recombination of minority carrier and from the edge of the depletion layer, it is considered that forming a length from the light emitting region to the front surface within 1/a (a is the absorption coefficient) is at least required.
That is, in a case of ZnTe, 1/a is 1 xcexcm, and it was found that forming the junction interface in a range of 0.3 to 2 xcexcm is most suitable, considering the depletion layer width and the diffusion length. As a result of experiments, when the diffusion length (diffusion depth) exceeded 2.0 xcexcm from the front surface, little green light emission was observed, while when it was below 0.3 xcexcm, leakage current increased and green-color was locally observed. Accordingly, the efficiency of the diffusion length of 0.3 to 2.0 xcexcm was able to be verified. A position of the junction interface can be confirmed by observing a cleavage surface of the substrate by SEM (secondary electron microscope) and with light and shade of the SEM image.
The sixth invention according to the present inventions was developed based on the above-described findings, and is an electro luminescence device comprising a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in a periodic table, wherein the electro luminescence device is produced by disposing a diffusion source including an element converting the substrate of a first conduction type into the one of a second conduction type on a front surface of the substrate; forming a pn junction in the vicinity of the front surface by thermally diffusing the diffusion source; and forming electrodes on both surfaces of the substrate, and wherein a depth of the diffusion is not less than 0.3 xcexcm and not more than 2.0 xcexcm from the front surface of the substrate.
Since the diffusion depth is limited as described above, the damping caused by the light absorption is reduced, and thereby the light emission intensity can be increased.
Preferably, the substrate may be ZnTe, ZnSe or ZnO. Further, the diffusion source may be Al, Ga, In, or alloy thereof.
According to the above-described means, the electro luminescence device of which a luminescence center wavelength is 550 nm to 570 nm can be obtained.
Next, the inventors or the like cut the light emitting diode into chips of determined size, packaged them with resin or the like, and evaluated the current-voltage characteristics (I-V characteristics) of the packaged light emitting diodes by applying forward current. As a result, it was found that there was much current flowing in a low voltage region or current flowing in reverse bias (leakage current), and the light emission efficiency was not so good.
Then, as a result of consideration for investigating of the causes, it was inferred that since the thermal diffusion is carried out such that the diffusion source is deposited over the entire front surface of the substrate, the pn junction interface is exposed to a cutting plane when the substrate is cut into chips, and process deterioration in the pn junction interface influences the increasing of the leakage current.
That is, it was considered that although a dicing saw is used for cutting the substrate into chips, the pn junction interface, which is exposed to the cutting plane, deteriorates in the cutting process by the dicing saw, thus the leakage current increases. Generally, it is often the cases that a way of etching the cutting plane is applied to removal of the processing deteriorated layer in the pn junction interface. However, in the case of the ZnTe substrate, there is no etchant for efficiently removing Te. Thus, because Te remained on the cross section after etching, it was difficult to sufficiently reduce the leakage current.
Therefore, a method other than the etching is required to reduce the leakage current. The inventors or the like investigated in more detail the causes of the occurrence of the leakage current, and as a result, found out that the leakage current at issue flowed through the pn junction interface in the cutting plane.
According to the result of the investigation, it was inferred that if the pn interface does not exist in the cutting plane of the substrate, it may be possible to control the leakage current after etching. Based on the inference, the diffusion source was deposited only on the portions other than portions to be cut, and the diffusion was performed, thereby the light emitting diode was manufactured by way of trial. The portions where the diffusion source was not deposited were cut by the dicing saw so that the light emitting diode was cut into chips, and then the I-V characteristics of the chips were evaluated.
As a result, it was able to be verified that the leakage current in the light emitting diode did not change after cutting compared to that before cutting, and the leakage current as in a case that the pn junction interface was exposed to the cutting plane did not increase, so that the light emission efficiency may be improved.
The sixth invention according to the present inventions was developed based on the above-described findings, and is an electro luminescence device comprising a compound semiconductor crystal substrate comprising a Group 12 (2B) element and a Group 16 (6B) element in a periodic table, wherein the electro luminescence device is produced by disposing a diffusion source including an element converting the substrate of a first conduction type into the one of a second conduction type on a front surface of the substrate; forming a pn junction in the vicinity of the front surface of the substrate by thermally diffusing the diffusion source; and forming electrodes on both surfaces of the substrate, and wherein the pn junction is formed so that a junction interface will not be exposed to the front surface of the substrate after etching at a cutting plane in the vertical direction.
Accordingly, the pn junction interface is not exposed to the cutting plane, so that the leakage current, which flowed through the pn junction interface in the cutting plane, is extremely reduced and the light emission efficiency can be improved.
With the substrate, the diffusion source may be partially deposited on a portion which is inside at a predetermined distance from the peripheral edge portion of the substrate, and the diffusion source may constitute one of the electrodes.
Preferably, the substrate may be any one of ZnTe, ZnSe and ZnO. Further, the diffusion source may be Al, Ga, In, or alloy thereof.
The electro luminescence device may be produced by disposing a mask on the substrate, the mask covering at least a portion through which a cutting means passes, the cutting means cutting the substrate into chips of an electro luminescence device, and the mask having opening in a portion where the diffusion source is disposed; depositing partially the diffusion source through the mask; forming a pn junction by thermally diffusing the diffusion source; forming electrodes on front and rear of the substrate; and thereafter cutting the substrate into chips by a predetermined cutting means at a portion which was covered with the mask and on which the diffusion source was not deposited. Preferably, the cutting means is a dicing saw, and the portion of the substrate through which the cutting means passes is formed in a width which is not less than double of a width of a blade of the dicing saw.